AN47

Issue 1 - August 2006

5

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© Zetex Semiconductors plc 2006

Resistor dimming

By connecting a variable resistor between ADJ and GND, simple dimming can be achieved.

GND

ZXLD1350

ADJ

GND

The current output can be determined using the equation:

I

OUT

0.08 R

S

⁄

() R

ADJ

×

R

ADJ

200k

+

()

------------------------------------------------

=

Note that continuous dimming is not possible with a resistor. At some point the shutdown

threshold will be reached and the output current reduced to zero. This can occur below 300mV.

Note that a 1M

The +/20% tolerance of the input resistance should also be understood. See table below:

Table 1

Rint nom.

k

Rint min.

k

Rint max.

k

nominal

% error

from

nominal

Due to

Rint min.

% error

from

nominal

Due to

Rint max.

1000

200

160

240

1.041

3.4%

-3.3%

500

200

160

240

0.892

6.1%

-5.7%

200

200

160

240

0.625

11.1%

-10.0%

100

200

160

240

0.416

15.4%

-13.3%